1. Field of the Invention
The present invention relates to a light-emitting device, particularly to such a device constituted by a light-emitting diode or light-emitting diode array.
2. Description of the Prior Art
Light-emitting devices constituted of P-N junction light-emitting diodes are used in optical printers which use a beam of light to record information, in image and bar-code reading systems which utilize the intensity of reflected light, and in optical communications devices which utilize optical signals.
These light-emitting diodes are constituted as a junction of P and N type semiconductor material to form a light emitting section which is made to emit light of a prescribed wavelength by applying a forwardly-biased voltage across this P-N junction. Usually a thin film of silicon-nitride or the like is formed adjacent to the light-emitting section which serves to protect the light-emitting section while also allowing the external emission of the light produced by the light-emitting section. The thickness of the thin film is selected with the object of maximizing the externally-emitted amount of light produced by the device. The following equation shows how such a thickness value is determined. EQU d=(.lambda./4n) . (2m+1) (1)
where d is the thickness of the thin film, m is zero or a positive integer, .lambda. is the wavelength of the light emitted by the light-emitting diode at room temperature and n is the refractive index of the thin film.
Here, the transmittivity T of the thin film will be EQU T=4n.sub.1 n.sup.2 n.sub.3 /(n.sub.1 n.sub.3 +n.sup.2).sup.2 ( 2)
where n.sub.1 is the index of refraction of the light-emitting section adjacent to the thin film, that is, of the P and N semiconductor, and n.sub.3 is the index of refraction of the air.
As described above, the diode emits light when a forward bias is applied across the P-N junction. Generally speaking, the energy efficiency of light-emitting diodes is low, no more than several percent, with most of the injected electrical energy being converted into heat in the light-emitting section. This heat raises the temperature of the light-emitting section, which reduces the light emission intensity and also shifts the center frequency of the emitted light towards the longer wavelength side of the spectrum.
FIG. 7 shows the spectral distribution obtained when using a P-N junction in which the P type semiconductor is a zinc-diffused GaAs.sub.0.4 P.sub.0.6 layer and the N type semiconductor is a GaAsP layer which includes tellurium. The horizontal axis is the wavelength .lambda. of the emitted light (nm) and the vertical axis is the relative intensity of the light emitted by the light-emitting diode device and the transmittivity of the silicon-nitride layer at a maximum diode output of 1.0. At room temperature, i.e. 25.degree. C., the center frequency of the emitted light is 660 nm, but as shown here, as the temperature of the light-emitting section rises, going to 30.degree. C., 35.degree. C., 40.degree. C. and 45.degree. C., the center frequency undergoes a shift to the longer wavelength side and there is a decrease in the relative intensity of the emitted light. With respect to FIG. 7, a value of 0.33 nm/.degree.C., which is typical for a light-emitting diode, has been used for the wavelength of the emitted light and the temperature dependency of the intensity of the emitted light, and the distribution is Gaussian with a peak width at half height of 30 nm.
FIG. 7 also includes transmittivity T values of the silicon-nitride layer relative to the wavelength .lambda. of the emitted light, which shows that transmittivity T decreases with the increase in wavelength .lambda.. If the index of refraction of the silicon-nitride layer is n=1.87, transmittivity T will be at a maximum when the index of refraction of the P and N semiconductor adjacent to the silicon-nitride layer is a typical n.sub.1 =3.5 and that of the air is n.sub.3 =1.0. Based on the use of equation (1) to determine the thickness d of the silicon-nitride layer to obtain the maximum transmission of light emitted by the light-emitting section, a thickness of d=265 nm is used, obtained using the values m=1, .lambda.=660 nm and n=1.87.
Thus, a drawback with conventional light-emitting diode devices is that the intensity of the emitted light is reduced by a rise in the temperature of the light-emitting section, the resultant lengthening of the wavelength spectrum tends to produce a drop in the transmittivity of the silicon-nitride thin-film layer, sharply reducing the intensity of the emitted light.
When such a light-emitting diode is used as the light source in an optical printer, image reader or other such optical communication apparatus, this decrease in light intensity degrades the image quality, increases the read-error rate and produces a deterioration of the signal-to-noise ratio.